UV editing + texture - change color of the PNG image (caption)
You'll find here a long and complete answer to your question.

To make it short, here is how you could organize your nodes if you want to use your image texture as a kind of mask between an emission node and your wave background: Plug the "Hello" Image Texture into the Factor of a Mix Shader, and plug what you want into the 2 sockets of your Mix Shader.